Blowing Fuse

On your rectifier picture, where is the yellow wire connecting?

The YLW wire is a waste of space, the centre bolt through the rectifier which has the Luca terminal (with the dodgy YLW wire) is the earthing bolt for the rectifier, this does not appear to be bolted to Earth? Usually you use the bottom of the tool tray. Which is removed in this picture.

When built this bike would have been 6V and of course Positive Earth. I would suspect it has remained Pos earth but converting to 12V is quite acceptable, but it should have a Zener diode fitted and retain the 3 wire RM19 alternator. The Ammeter won't work if there is no battery to charge! Using 'the Bike' without a battery is not an issue, but beware you have no stop light or horn, the zener if fitted could fail, and if travelling with a lot of engine revs on using the lights might blow bulbs.

Ascertain the rectifier polarity (they are mostly marked) throw away the dodgy YLW wire, remount the rectifier and refit the tool tray, check the presence of a Zener, and refit a new battery (Positive earth) then if the lights work start the engine and you might even have a working ammeter. Further information available but recommend a direct connection to me.

If the zener is not secure the wire can earth and blow the fuse .

Alistair, 

Forgive me if I have misunderstood or read your post wrong, but if you connect the negative from the battery to the mounting point on the tool tray, this will short circuit and blow any fuse.

What I do in situations like yours is a make up a temporary "fuse" using an old headlamp globe and 2 alligator clips to replace the fuse . Or better still connect between the positive terminal and red wire. When a short circuit occurs the globe will glow and you will save a fuse and possibly some melted wire.

In answer to where to look. I would go inside the headlight shell. Maybe the feed to the ammetre has come off? The headlight is a happy hunting ground for loose connections and bad insulation. (As is under the tank and seat on English bikes)

Good luck.

Don Anson Melbourne Australia 

Two wires on the Zener?. 

... I'd replace that rectifier with a modern solid state encapsulated one. They're widely avaikable and less than £10.

The SS is a magneto bike therefore no ignition switch (normally). The lights will work off the battery (until flat). Yes knackered battery could allow lights to work but only with engine running.

Ian be careful about buying a rectifier from just anywhere. There is very many encapsulated ones on the market but only a few are up to motorcycle spec. As per I sell.

The most likely cause for the fuse to blow and the rectifier and even the zener to fail is fitting the battery reverse polarity ie negative earth. If the rectifier had failed (short circuit) in time the battery could have dried out/failed, but it takes time. Once the battery has dried out/failed then the rectifier will not be affected, but continued use of higher revs with the engine (at speed) might cause the zener to fail.

Although it may have a Lucas logo it's now supplied by Wassell, that well known purveyor of junk going back to the 1970s at least. I'd still go for a solid state job - fully accepting Al's points.

Wassell have NO technical back up for the customer-buy at your peril! A O Services only sells what I understand. There is NO Lucas, the Real Lucas died in the 1980s, how many times do we need telling.